If cairo-doc (default on vl7.0 std and soho) just right click on the dock go to cairo-doc and exit.

Only one instance of the doc will quit at a time, leaving you with the one you need.

Also, note if you have choosen to save sessions when you log out or shut down, you will also end up with multiple docs, as you will get one from the saved session and one from auto-start.After exiting any excess sessions either remove the auto start line for cairo-doc or disable the save session when you log out or shut down.